It is frequently necessary to provide a clear indication of the instantaneous value of the frequency of a signal produced by a sweep signal generator. Such a generator produces a signal whose frequency varies in a repetitive, periodic manner. The frequency variation may occur in a sinusoidal manner with time, for example, sweeping alternately above and below a predetermined center frequency value. A common method of indicating the frequency variation of such a signal is by means of a series of frequency markers on a cathode ray tube display, arrayed in line, with the spacing between successive markers indicating equal increments of frequency. It the sweep signal is displayed on the same cathode ray tube, then it is possible to determine the frequency of the sweep signal at any point in its progress from a minimum frequency value to a maximum frequency value, by observing the relationships between the frequency markers and the displayed sweep signal. In prior art embodiments of such frequency marker systems, a large number of markers are simultaneously displayed. It is therefore difficult to determine rapidly the actual value of frequency to which a particular marker corresponds.
With the present invention, this difficulty which is encountered with such prior art frequency marker systems is eliminated by providing means whereby a particular frequency marker, or pair of frequency marker symmetrically disposed with respect to the center frequency marker, can be immediately selected by an operator, through actuation of a switch. Since this switch may be provided with indications which show the frequency value for each of the frequency markers selected at each setting of the switch, it is apparent that selection of a particular frequency marker, or determination of the frequency of the closest frequency marker to a particular portion of the swept signal, can be readily achieved by the present invention. In addition, the present invention can be readily implemented by relatively simple modification of a frequency marker system of conventional design.